1. Field of the Invention
The invention relates to a device for the projection printing of masks onto a workpiece, particularly onto a semiconductor substrate for the production of integrated circuits, the patterns of the masks being imaged on a photosensitive layer of the workpiece via a projection lens by exposure light, and mask and workpiece being aligned relative to each other by imaging alignment patterns of the mask by means of adjustment light through the projection lens on adjustment marks of the workpiece and by using an inaccuracy of this imaging observed through the projection lens for the formation of an adjustment signal.
2. Description of the Prior Art
In the production of integrated circuits it is necessary that a number of masks having different circuit patterns are sequentially imaged on exactly the same area of the workpiece. After exposure and development the photosensitive layer serves for covering certain regions of the workpiece during chemical and physical processes, e. g. etching- and diffusing processes, carried out between the consecutive exposures. In the manufacture of integrated circuits high demands are made to accuracy.
Tolerable displacements of the consecutive images of the circuit patterns are, for example, less than 1 .mu.m. In order to obtain such accuracy the circuit patterns on the masks are imaged on the workpiece, usually by means of a projection lens, reducing by the factor 10. Before exposing a workpiece or a portion of said workpiece which have already been provided with circuit elements it is necessary to align alignment patterns of the mask relative to adjustment marks on the workpiece by adjusting the mask or the workpiece, in order to obtain the desired overlay of the circuit patterns.
For the alignment process the respective adjustment mark areas on the workpiece and the areas of the alignment patterns on the mask are imaged into one another by means of the projection lens, the relative deviation being visually or metrologically determined. Positioning commands for the adjustment mechanism are deduced from the deviation.
In order to preserve the adjustment marks for future exposure processes, adjustment is frequently made with light having a wavelength to which the photosensitive resist is not sensitive. It may further be provided (cf. DE-OS 28 45 603) to arrange the alignment patterns of the mask, which may be in the shape of rectangular windows, and the adjustment marks on the workpiece, onto which the image of the alignment patterns shall be centered, at points non-conjugated in respect of the projection lens. In this case, auxiliary optical means are provided in the path of the adjustment light during adjustment.
Gauging is a problem in any kind of adjustment system: it must be ensured that the adjustment means judge the relative position of mask, projection lens and workpiece as optimal in which the overlay of successively applied circuit patterns in the exposure light is actually as accurate as possible. A systematic difficulty thereby results from the advantageously used adjustment light which is non-aggressive in respect of the photosensitive resist, as the projection lens is, of course, adjusted in respect of the exposure wavelength and not of the adjustment wavelength. The arrangement of auxiliary optical means, when using non-conjugated marks, also reduces the degree in which the evaluation result of the adjustment means can be considered representative for the actual quality of the picture overlay.
Gauging of the adjustment means is presently effected by successively moving the displacement stage carrying the workpiece into various positions and exposing said workpiece, the corresponding adjustment signal being noted. The exposed chips are developed and examined in respect of their quality. The adjustment signals which correspond to that position in which the best result has been obtained are used as set values for further exposures. It is obvious that such a complicated method for the adjustment of the adjustment means is justified for the first use of the device. It cannot be expected that the person using the device compensates the zero shift occuring in the course of time by means of this method. Moreover, the adjustment marks on the workpiece are not always at the same points so that a new adjustment of the adjustment means, which should be possible in an easy manner, is required, when switching between types of semiconductor substrates having different mark arrangements.
In order to obtain congruent imaging of a circuit pattern on a pattern which has already been imaged it is not sufficient to center the new pattern with the old one by displacing the mask or the workpiece in their own (X-Y) plane. It is further necessary to bring the workpiece into a position in which its entire surface to be exposed is precisely focused, which is effected by displacing the workpiece in Z-direction by means of three adjustment means not lying on a straight line. Congruent imaging of the patterns is, thus, not yet ensured, however, as finally a constant magnification scale for all imaging processes must be guaranteed, i.e. two points of the mask plane positioned at a certain distance from each other must always be imaged at the same reduced distance in the plane of the workpiece. As the projection lenses which are now in use are telecentric systems, magnification actually depends only on the distance of the mask from the projection lens but this distance can easily change when a new mask is inserted or through temperature influences so that the maintenance of the magnification scale forms an essential part of the adjustment process.